The Sega Game Gear is a handheld game console which was Sega's response to Nintendo's Game Boy. It was the third commercially available color handheld console, after the Atari Lynx and the Turbo Express.

Work began on the console in 1989 under the codename "Project Mercury", following Sega's policy at the time of codenaming their systems after planets. The system was released in Japan on October 6, 1990, in North America, Europe and Brazil in 1991, and in Australia in 1992. The launch price was $149.99. Sega dropped support for the Game Gear in early 1997.

The Game Gear was not very popular in Japan, where it was released to a generally apathetic audience, with build quality issues plaguing it early in its service life. Another problem was battery life; while better than earlier color backlit systems, its 5 hour battery life (using alkaline batteries) still wasn't as good as the Game Boy (due to that system's monochrome screen, lack of a backlight, and less powerful hardware). Battery life was a much bigger issue before handheld systems had built-in rechargeable batteries: gamers needed either a constant supply of six AA batteries, or a rechargeable battery pack that was sold separately.

The Game Gear was more expensive than the Game Boy ($149.99, versus $89.99 for the Game Boy). The significantly larger price tag contributed to driving away potential Game Gear buyers.

When first launched in America, a memorable TV advertising campaign was used to promote the system as superior to the Game Boy. One commercial featured a dog looking back and forth at both portables, with a narrator saying, "If you were colorblind and had an IQ of less than twelve, then you wouldn't care which portable you had. Of course, you wouldn't care if you drank from the toilet, either."An advertisement was shown in black and white, with players milling about aimlessly in a dark void, playing Game Boys. A lone rebel appears with a Game Gear, cuing the narrator's comment of "The Sega Game Gear: Separates the men from the boys." Another showed a gamer (Ethan Suplee) hitting himself in the head with a rigid, dead squirrel in order to see color on his Game Boy. When the Game Boy began to appear in different colors, Sega's ad ridiculed it by showing the Game Boy disguised in loaves of bread. Another ad from that era featured a professor explaining that though the Game Boy now was available in bright colors, the graphics were still monochrome, and therefore Game Gear was still superior. Although Sega was rather proud of these original marketing campaigns, it may have backfired since many gamers - loyal to their existing Nintendo handhelds - saw the ads as offensive, condescending or even patronizing. Negative advertising may have also been detrimental since it implied that the Game Gear was in second place (as indeed it was). However, less offensive advertising included the phrase SEGA does what Nintendon't, but even that phrase did not discourage Nintendo fans from buying the Game Boy instead.

Although its color backlit screen and ergonomic design made it technically superior to the Game Boy, the Game Gear did not manage to take over a significant share of the market. This can be blamed partly on the perception that it was too bulky, and on its somewhat low battery performance: the device required six AA batteries, and the backlit screen consumed these in approximately five hours (six on the later versions), compared to 10-14 hours of four-AA battery lifespan for the Game Boy. External and rechargeable battery packs were sold to extend the devices' battery life. At that time, rechargeable batteries had strict limitations (e.g. the batteries needed to be discharged before being recharged). Ni-Cd batteries were the mainstream type of rechargeable batteries during that era, and Ni-MH and Li-Ion batteries would not become mainstream until after the Game Gear was phased out.

However, Sega's biggest problem was that it failed to enlist as many key software developers as Nintendo, so the Game Gear was perceived as lacking as many games. Although it was a moderate success, the Game Gear did not manage to achieve the commercial success that Game Boy did, in that when it went off the market it was not replaced by an immediate successor. The Game Gear, however, did better than other portable systems that tried to compete with the Game Boy, such as the preceding Atari Lynx. The Game Gear did suffer from some of the same key problems that plagued the similar Lynx, though Sega did somewhat better than Atari due to more titles and a stronger marketing campaign. In the end, the Game Gear gained most of its sales by pushing the Lynx out of the market rather than eating into the Game Boy's dominant share.

Over 250 titles were released worldwide for the Game Gear, although at the time of the console's launch there were only six software titles available. Sega made sure that a wide variety of video game genres were represented on the system, in order to give it a broad appeal. Prices for game cartridges initially ranged from $24.99 to $29.99 each. They were molded black plastic with a rounded front for convenient removal. The original Game Gear pack-in title was Columns. It was similar to the Tetris cartridge that Nintendo had included when it launched the Game Boy.

Popular titles included Sega's own series, notably Sonic the Hedgehog, Disney movie extensions, such as The Lion King, and 3rd-party developer games like Mortal Kombat.

In an unusual step, Sega decided not to region encode Game Gear cartridges, meaning that any system could play any games regardless of the country they were released in. This practice helped to make the console popular among import gamers.